All that is because they first believed and then disbelieved, and therefore a seal was set on their hearts; as a result they understand nothing.
इस सबब से कि (ज़ाहिर में) ईमान लाए फिर काफ़िर हो गए, तो उनके दिलों पर (गोया) मोहर लगा दी गयी है तो अब ये समझते ही नहीं